Topcoat coating compositions, predominantly curable (thermosettable) topcoat coating compositions, are widely used in the coatings art, notably in the automotive and industrial coatings industry. It is important for a topcoat to be durable and to protect the coated article or to aid underlying coating layers in protecting the coated article. A primary function of a topcoat, however, is to provide aesthetic appeal. The automotive industry has made extensive use of these coatings for automotive body panels, both as metallic colors and as straight shade colors. Color-plus-clear composite coatings are particularly useful as topcoats that provide exceptional gloss, depth of color, and distinctness of image.
Metallic colors are generally defined in the art as colors having gonioapparent effects. For example, the American Society of Testing Methods (ASTM) document F284 defines metallic as "pertaining to the appearance of a gonioapparent material containing metal flake." Metallic colors thus include special flake pigments, which may be metallic flake pigments like aluminum pigment or pearlescent flake pigments including treated micas like titanium dioxide- or iron oxide-coated mica pigments, to give the coatings a different look when viewed at different angles. Traditional metallic colors are lighter when viewed at an angle closer to the specular, commonly referred to as the flash angle, and darker when viewed at an angle significantly farther from the specular, commonly referred to as the flop angle. For example, metallic colors would appear lighter in value when viewed at angles of 5 to 15 degrees off specular and darker in value when viewed at angles 75 to 110 degrees off specular. This value difference, caused by the presence of the flake pigments in the coating, gives the coated article a generally metallic look. In addition to a value difference, a change in chroma or hue of the color may be observed in moving from a view at an angle close to specular to a view at an angle significantly off specular. Colored aluminums have been used in metallic coatings to modify the gonioapparent effects of a coating in this way or to increase the chroma of the coating.
Solid colors (or straight shade colors), on the other hand, are intended to have the same or nearly the same color from any angle and do not have an apparent metallic appearance. Straight shade colors have in the past not included any flake pigments at all. For example, ASTM E284 describes a solid color as "not containing flake or gonioapparent pigments." The solid colors rely on color pigments to make high chroma colors like reds, blues, oranges, and yellows, as well as lower chroma versions in many different hues. In recent decades, many of the best pigments for color-fastness and high chroma of color are no longer used in automotive quality coatings because of potential health concerns or regulatory considerations. Notably, cadmium, lead, and chromate pigments are generally no longer used. The substitutes for these pigments have shortcomings that have made development of high chroma colors or clean pastel colors difficult. The inorganic pigment candidates that are currently in use have good durability, but insufficient color intensity. In order to obtain high chroma colors, the more color-intense organic pigments are typically used. While the organic pigments provide sufficient chromaticity, they often do not provide sufficient opacity (also called hiding) for normal automotive or industrial coating requirements. The organic pigments, therefore, must be included in the coating at higher than desired levels and, usually, other more opaque pigments, which detract from the chroma, must also be employed to achieve a high chroma coating with sufficient hiding at the specified filmbuild.
While it is thus often necessary to include more pigment in the composition in order to obtain the desired color intensity, a lower pigment-to-binder ratio would be preferred for physical properties of the coating. Additionally, because the pigment combination at the chosen chroma may provide insufficient hiding, the coating composition may need to be applied at higher than ideal filmbuilds, resulting in added cost and potential problems such as sagging, solvent popping, and less than optimum properties for repairing defects. As evidenced by the above ASTM definition of a solid color, the metallic or mica flake pigments have generally only been included in automotive coatings when the gonioapparent effect or metallic effect was desired. Small amounts of uncolored aluminum flake pigment have been incorporated into a white basecoat to reduce, in part, the significant amount of titanium dioxide required for color and hiding. The addition of aluminum pigment was undesirable in that it reduced the chroma of the white basecoat, similar to other opacifiers such as carbon black.
It would be desirable, therefore, to be able to increase the opacity and/or improve the chromaticity of a straight-shade color at specified filmbuilds to avoid these problems. It would likewise be desirable to reduce the total amount of pigment required in order to provide a lower-cost coating having better physical properties.